In general, X-rays radiated to an object to be imaged are attenuated depending on an X-ray attenuation coefficient of a substance located on a radiating path by photoelectric effect or Compton scattering. X-ray imaging is a radiation photographing method using a penetration characteristic of X-rays, and image information of an internal structure of the object is provided based on the amount of accumulated attenuation in a process of penetrating through the object. Generally, an X-ray imaging device includes an X-ray source radiating the X-rays, an X-ray detector detecting the X-rays penetrating through the object while allowing the X-ray detector and the X-ray source to face each other with the object located therebetween, and an image processing unit forming an X-ray image by using a detection result of the X-ray detector.
The penetration image showing overlapped the internal structures in a direction of the X-rays radiated to the object is obtained, and a three-dimensional image of the object may be obtained by applying image processing, such as volume rendering and/or surface rendering, to multidirectional X-ray images for the object. A variety of tomograms may be obtained depending on a random position or random direction desired by a user by using the three-dimensional image of the object.
Recently, due to developments in semiconductor and data processing technologies, X-ray imaging has been rapidly replaced with Digital Radiography (DR) using a digital detector, and imaging methods have been variously improved.
FIG. 1 is a view illustrating an X-ray panorama image mainly utilized in the dental field. The X-ray panorama image displays teeth arrangement spread in a penetration image according to a predetermined trace of a dental arc. Accordingly, an entire teeth arrangement may be easily understood as a single image, and thus the X-ray panorama image is used as the most familiar standard image by dentists. However, an accuracy level of length information of a general panorama image is low, and there is a limitation of a projection image, such as an overlap of teeth and blurring by the cervical spine. FIG. 2 is a view illustrating an example, commonly used in the dental field, of an X-ray computerized tomography (CT) image for a head. An object to be imaged, for example, an entire head to be imaged, is located in a field of view (FOV), and a penetration image is obtained in each of imaging positions by performing X-ray imaging. Further, a three-dimensional image is produced by performing image processing, and a tomogram at a selected position from the three-dimensional image is produced and displayed. Accordingly, the three-dimensional image for the entire FOV and tomograms according to a position and a direction desired by a user may be accurately and clearly displayed. Due to this, the three-dimensional image is utilized when high-degree accuracy is required, for example, an implant procedure. However, a general three-dimensional X-ray image device is problematic in that the amount of radiation exposed to a patient is relatively high and an expensive large area detector is required.
When imaging for producing the three-dimensional X-ray image according to the related art, it is necessary that an entire object to be imaged is located within the FOV. Thus, the large area detector compared with a detector for imaging the X-ray panorama is required. For example, a case, in which a CT image is obtained in a state of positioning the object within the FOV of a first height t1 and a first width w1 by using the X-rays formed in a cone beam shape mainly used in a dental field, is described below. In this case, when a second height t2 of the detector is a value obtained by multiplying magnification by the first height t1 or more (t2 ≥ a value obtained by multiplying magnification by t1) and a second width w2 of the detector is a value obtained by multiplying magnification by the first width w1 or more (w2 ≥ a value obtained by multiplying magnification by w1), the X-rays for an entire area, which have penetrated the FOV, may be received.
Meanwhile, if a vertical shaft longitudinally parallel to the FOV is the same as rotary shafts of an X-ray source and the detector and if the X-ray source and the detector are rotated 360 degrees based on the rotary shafts, a relatively small detector may be used in case of a half beam method, capable of decreasing the second width w2 of the detector to a value obtained by multiplying the maximum magnification by w1 divided by 2, is used. However, under any methods, at the time of X-ray imaging for producing a three-dimensional image, the area of the detector producing a three-dimensional image should be much larger than the area of the detector for X-ray panorama imaging. When it is required to obtain the X-ray panorama image and the three-dimensional image that have equal heights to a height of the object, an X-ray panorama image detector is formed in a slit shape having a width ranging from only 5 to 10 mm. Meanwhile, an X-ray detector for producing a three-dimensional image is formed in a square shape whose width is approximate to a height thereof. Generally, since a price of a detector is considerably increased depending on an area thereof, it is not possible for an X-ray device for producing a three-dimensional image to avoid cost increase caused by a large area detector, thereby increasing in equipment cost. Further, when an area of a sensor is increased, the weight of equipment is increased. Thus, it is problematic in that a size of equipment should be larger because a focal spot to detector distance (FDD) is increased as well so that the FOV can be ensured.